Cells and organs of the immune system By: Dr Iman Hussein Shehata Professor of Medical Microbiology and Immunology

By the end of this lesson the student is expected to: 1- List organs of the immune system 2- Describe the function of each organ 3- Enumerate cells of immune system and identify the site of each cell 4- Describe the function of each cell in immune system 5- Compare between the two branches of immune system

What is Immunology??? Immunology is the science that studies the defensive mechanisms of the host against **foreign material or infectious agent and **distinction of self from non-self.

Functions of Immune System Has the ability to distinguish ‘self’ from non-self Inactivates/destroys foreign substances *Foreign molecules *Molecules present in viruses, bacteria, parasites. *Microorganisms and other cells (virus-infected cells, cancer cells, cells of transplanted organs)

Organs of the immune system 1- Primary (central) lymphoid organs: in which immune cells develop, and mature 2- Secondary (peripheral) lymphoid organs: in which cells of the immune system meet, interact, and perform their functions

ANATOMY OF THE IMMUNE SYSTEM The immune system is localized in several parts of the body immune cells develop in the primary organs - bone marrow and thymus (yellow) immune responses occur in the secondary organs (blue) The organs of the immune system are color-coded on this slide. yellow = primary organs - bone marrow, thymus blue = response organs  

Primary (central) lymphoid organs: 1- Bone marrow: contains pleuripotent stem cells which have the ability to differentiate into - red blood cells, - platelets - cells of immune system.

Maturation of the cells of the immune system

Cells of immune system are either: Circulating in blood: -Granulocytes:neutrophils, eosinophils,basophils, -Monocytes -Lymphocytes (three types; B and T lymphocytes, and natural killers) Residing in tissues: Macrophages dendritic cells mast cells Present in tissue and blood: lymphocytes

2- Thymus: The thymus is bi-lobed structure located, in front of the heart and behind the sternum. maturation of T lymphocyte T cells learn to differentiate between "self" (the body's own cells) and "nonself" (foreign objects, organisms, or diseased cells). If a T cell recognizes a self-cell as foreign, the cell will be destroyed.

Secondary (peripheral) lymphoid organs Cells of the immune system are found in blood and in a variety of organs called the secondary lymphoid organs *spleen *lymph nodes *Mucosa Associated Lymphoid Tissue

1-Spleen Contains : Lymphocytes (B cells, T cells and natural killer) macrophages and dendritic cells. is an immunologic filter of the blood that removes any foreign material or damaged cells.

Spleen Largest accumulation of lymphoid tissue Abundant phagocytic cells— defense against antigens in blood Site of destruction of aged erythrocytes. Production site of activated lymphocytes which are delivered to the blood. THUS, an important blood filter and antibody-forming organ

Spleen

Spleen

**remove any foreign material** Lymph nodes: the same immune cells found in the spleen. These specialized cells sample the incoming lymph (fluids drained from tissues and eventually return to the blood for circulation) to **remove any foreign material**

Lymph Node

Cells of Lymph Node Lymphoid cells Macrophages and other phagocytic antigen processing cells Lymphatic and vascular endothelial cells fibroblasts responsible for lymph node supporting framework.

Lymphatic System

Immune cells travel back and forth from the blood and lymph circulatory systems and interact with extra-vascular tissues in the process--extravasation muscle

Lymph Nodes Functions : Filtration of particles and microorganisms to keep them out of general circulation. Interaction of circulating antigens in lymph with lymphocytes to initiate immune response. Activation, proliferation of B lymphocytes and antibody production. Activation, proliferation of T lymphocytes.

Lymphatic Capillaries

Structure of Lymph Node

3-Mucosa Associated Lymphoid Tissue (MALT): Approximately >50% of lymphoid tissue in the body is found associated with the mucosal system. Mucosa associated lymphoid tissues are either in the form of : a- Diffuse collections of lymphocytes and macrophages as the lymphoid tissue lining the intestinal tract, the respiratory tract, and the genitourinary tract. b- Clearly organized tissues with well-formed lymphoid follicles as tonsils in the oropharynx, and Peyer’s patches in the intestine and appendix.

Cells of the immune system A- Cells of immune system circulating in blood 1-Neutrophil most abundant population of circulating white blood cells. identified by: segmented nucleus and granular cytoplasm. cytoplasmic granules are filled with enzymes as lysosymes, collagenase, elastase and defensins. These contents aid in digestion of phagocytosed material. first phagocytic cells that normally encounter an infection in blood and may migrate from the bloodstream into tissue spaces. Increased neutrophils are detected in acute infections of bacterial origin.

Neutrophil phagocytosing S. pyogenes, the cause of strep throat CELLS alive! Streptococcus pyogenes, the pathogen that causes strep throat is the yellow, bead-like structure. Human neutrophils are WBCs that arrive quickly at the site of a bacterial infection and whose primary function is to eat and kill bacteria. This neutrophil is ingesting Streptococcus pyogenes

clinical manifestations of allergy 2-Eosinophils : Present in blood and may migrate from the blood stream into tissue spaces. have a two-lobed nucleus and granular cytoplasm. The content of the granules when released plays a role in damaging parasites clinical manifestations of allergy

3-Basophils : Basophils have bilobed nucleus and a cytoplasm contain basophilic granules with active mediators, such as histamine, and enzymes, such as peroxidases and hydrolases. These pharmacologically active amines when released play an important role in allergic reactions . Like all circulating granulocytes, basophils can be recruited out of the blood into a tissue when needed.

4-Lymphocytes: Lymphocytes can be divided into three populations: T cells, B cells, and natural killer cells (NK, cells). B lymphocytes: The activated B cell is called a plasma cell that begins to secrete large quantities of antibodies. These antibodies can directly neutralize toxins and viruses, stimulating an efficient phagocytic response (opsonization)

Opsonization

T Lymphocytes: T lymphocytes leave the bone marrow to mature in the thymus gland. There are two types of T cells; T helper cells secrete proteins called cytokines with various effects on other cells and T cytotoxic cells directly kill malignant cells or viral infected cells.

Natural killer (NK) cells are a small population of large, granular lymphocytes. The major NK cell function is to destroy *malignant cells *virally infected cells *cells infected with intracellular bacteria

5-Monocytes : circulate in blood They are phagocytic cells with a bean shaped nuclei and finely granular cytoplasm. reside in tissues giving rise to macrophages

B-Cells of immune system residing in tissue 1-Macrophages: present in different tissues. They are particularly concentrated in the lung (alveolar macrophages), liver (Kupffer cells) and lymph nodes and spleen where they are well placed to filter off foreign material   ingest and kill bacteria, viruses, and other foreign particles.

Macrophage and E. coli pink = macrophage yellow = bacteria; note rod-like structure of E. coli ©Dennis Kunkel Microscopy, Inc., www.DennisKunkel.com

2-Dendritic Cells: The word "dendritic" means "branched like a tree." It comes from the Greek "dendron" (tree). Dendritic cells are present in the skin and mucous membranes of the nose, lungs, and intestine where they readily contact invading pathogens.

Like monocytes and macrophages, dendritic cells are phagocytic cells. They are migratory cells travel to lymph nodes or spleen where they activate certain lymphocytes to go out and attack any similar foreign materials in the body.

3- Mast cells: Mast cells are fixed in tissues especially under the skin and in the mucosa of the respiratory and gastrointestinal tracts. Mast cells are similar to basophils in many ways. Their cytoplasm contains basophilic granules with active mediators, such as histamine, and enzymes. These mediators play an important role in allergic reaction

Innate and adaptive immunity The immune system is divided into two major components: innate immune system and the adaptive immune system. Defense against microbes is mediated by early reaction of innate immunity and later response of adaptive immunity.

Characters of innate immune system 1- first defensive mechanism of the body. It is ready to be activated upon the first signs of infection. 2- Nonspecific to the type of organism it fights. It reacts to structures common in microbes and does not distinguish fine difference between foreign substances. 3- It has no ability to remember encountered pathogens. 4- It has the same magnitude in response even in repeated infections with same microbe.

Characters of adaptive immunity: 1- It provides protection when innate immunity fails to eliminate infection. Both innate and adaptive immunity collaborate against any invading pathogen 2- Specific response (distinguish between different even closely related microbes and molecules). 3- It has the ability to remember encountered pathogens. A second infection by a pathogen is met with a more rapid and vigorous immune response.

Summary Organs of immune system Primary lymphoid organs Secondary lymphoid organs Spleen Lymph nodes Mucosa associated organs Bone marrow: in which cells of immune system are produced Thymus gland: In which T lymphocytes mature in which cells of immune system are collected to perform their function

Cells of the immune system are either: Circulating in blood: neutrophils, eosinophils, basophils, monocytes and lymphocytes (there are three types of lymphocytes; B lymphocytes. T lymphocytes and natural killers) Residing in tissues: Macrophages dendritic cells and mast cells Present in tissue and blood: lymphocytes

Difference between innate and adaptive immunity Innate immunity Takes few days to react Immediate Time of reaction React specifically against the microbe initiating it React against common structure in microbes i.e non specific Specificity Present. subsequent exposure to the same stimulus gives higher response Absent, subsequent exposure to the same stimulus gives the same response Memory

Thank you